Smart Clocking Techniques Extend Battery Life of Wearables
Dec 2, 2015
Jehangir Parvereshi Sr. Manager, Customer Engineering
SiTime Corporation
Today’s Wearables: Space Sensitive & Battery Driven
Wearable electronics are designed to: – Fit in ever-shrinking PCB real-estate – Collect and send data in short bursts – Return to the lowest power state – Stay in the lowest power state as long as feasible – Run for days on a small capacity, small footprint battery – Optimize current drain in a cyclic sleep scenario
Battery life is dictated by the coulombs consumed per cycle – Coulombs = Iactive.TON + Isleep.Tsleep
– Expressed in µC or mAH
Active State
Sleep State
Active State
Wake-up
TimeShut-down
Time
Tsleep TON
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Wearable Architecture
Major power consumption contributors – MCU – Wireless radio
Sleep Clock
RTC/WD Clock
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Which RF Protocol is a Power-miser?
From: A. Dementeyev, S. Hodges, S. Taylor and J. Smith, "Power Consumption Analysis of Bluetooth Low Energy, ZigBee, and ANT Sensor
Nodes in Cyclic Sleep Scenario," in IWIS, Austin, 2013.
The Bluetooth Low Energy (BLE) protocol has the lowest current drain • BLE has optimized radio power, bit-rate and connection time
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BLE Power Savings Model
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Sleep Clock Accuracy (SCA) & BLE Power
Master Slave
Initiating_LL_PDU SlaveSCA
setting:
0 to 500 ppm
Bluetooth Core 4.0, Vol 6 Specification Conn_Update_PDU
Slave negotiates link
parameters to (1)
determine Sleep Time,
and (2) to receive Master
SCA setting
Slave determines wake
up time based on the
combined MasterSCA
and SlaveSCA settings
and Sleep Time
• Average power is directly proportional to the ratio of “ON” time to “Sleep” time
• Early ON time (ΔT) to accommodate inaccurate sleep clock causes power penalty
• ΔT = (SLEEP CLOCK ACCURACY) * (SLEEP TIME)
Early ON Time
SLEEP ON
SLEEP TIME TON
ΔT
ON
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SCA & Window Widening in BLE Core 4.0
ΔT = (SLEEP CLOCK ACCURACY) * (SLEEP TIME)
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BLE Power Savings Evaluation System
32768 Crystal
USB Powered CC2540 BLE
MASTER SLAVE
Current waveform reflects Connection event profile
PC
USB
CC2541 Key FOB Slave
TI CC2540 TI CC2541
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BLE Window Widening Details
Sleeping Sleeping
Wake up Tx
ON Time
RX
Window Post-process Pre-process
Rx window width is proportional to (masterSCA + slaveSCA)
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Measured ‘ON Time’ vs. Sleep Time
masterSCA + slave SCA= 80 ppm For Sleep Time = 4s; ON Time = 3.6 ms
ON Time = 5.43 ms
Sleep Time (ms)
ON Time (ms)
100 2.9
2000 3.2
4000 3.6
8000 4.3
16000 5.2 ON Time = 3.6 ms
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BLE Power Savings with a 32 kHz TCXO
200 ppm quartz crystal
5 ppm TCXO
30% power savings going from ±200 ppm to ±5 ppm SCA
Quartz RX widening unacceptable at sleep time > 8s
2 µA Sleep current
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BLE Power Savings Test Results
µPower 32 kHz TCXO vs. 32 kHz crystal – µPower TCXO supports tighter SCA of < 5 ppm for maximum
power savings across operating temperature
• Typical 32 kHz crystal solution can only support SCA > ±200 ppm
– The actual frequency stability of Master and Slave sleep clocks must match the tighter programmed SCA settings to avoid link time-out
– A 32 kHz TCXO supports longer sleep times necessary to maximize power savings
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Overview of Clock Sources
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Characteristics of Clock Sources in Wearables
Device Today’s Typical
Clock Sources
Frequency
Range (MHz)
Current
(uA)
Resume
Time (ms)
Frequency
Stability (ppm)
MCU
RC MHz on-chip XO 0.1 to 48 5/MHz** 0.02 ±2500*
RC kHz on-chip XO 0.032768 0.12 0.15 ±2500*
Ext XTAL + on-chip MHz XO 4 to 48 34/MHz** 3 ±40
Ext XTAL + on-chip kHz XO 0.032768 0.5** 2000 ±200
BLE
RC MHz on-chip XO 16 3/MHz** 0.02 ±10000*
Ext XTAL + on-chip MHz XO 16 400** 3 ±40
Ext XTAL + on-chip kHz XO 0.032768 2** 0.5** ±250
* Calibrated with 32 kHz crystal oscillator; drifts with VDD, and temperature; CPU calibration power additive ** At 25⁰C; increases with temperature
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Clocking Characteristics for Next Gen Wearables
Device Next Gen
Clock Source
Frequency
Range (MHz)
Current (uA) Resume
Time (ms)
Frequency
Stability (ppm)
MCU
High Speed 4 to 24 10/MHz* 2 ±50*
Low Speed 0.032768 1* 200 ±5*
BLE
High Speed 16, 24 10/MHz* < 1 ±40*
Low Speed 0.032768 1* 200 ±5*
* Across operating temperature, VDD, and aging
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µPower MEMS Clocks
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Architecture of a µPower MEMS Oscillator
• < 100 ppm XO, ± 5 ppm TCXO frequency stability over temp • 1 Hz to 26 MHz programmable clock output • Environmental resiliency 30x better than quartz
MEMS Oscillator
Sustaining
Circuit
Frac-N
PLL
Charge
Pump
Temperature
To
Digital
OTP
Memory
Temp
Comp
Dividers,
Drivers,
I/O
CTRL
CLK
Programmable Analog IC
MEMS Resonator
1.5 mm
0.8
mm
524 kHz
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32kHz MEMS XO & TCXO vs. Quartz XTAL
Features SiT1532 / 52 Quartz XTAL
Package Footprint w/ Load Caps
1.2mm2 (80% smaller)
5.5mm2
Load Capacitors No Yes
Load Dependent Start-up No Yes
Bypass Caps No NA
32kHz CLK
XIN
XO
MCU, BLE, or Chipset PMIC
SiT1532 XO / SiT1552 TCXO
Quartz XTAL Solution
XIN
XO
MCU, BLE, or Chipset PMIC
Osc I/O Osc I/O
Vdd
XTAL+Caps
1.2mm2 total footprint 5.5mm2 total
footprint
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Industrial Temp Specification
MEMS XO 100 PPM Over Temp
2x More accurate than quartz XTAL
MEMS TCXO ±5 PPM Over Temp
30x – 40x more accurate than quartz XTAL
Quartz XTAL -160 to -200 ppm
Over Temp
Measured Measured
µPower MEMS 32 kHz XO & TCXO for Wearables
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µPower MHz MEMS Oscillator for Wearables
MEMS core power: 60 uA
Smallest footprint: 1.2mm2
Any frequency: 1 to 26 MHz
MCU Operation Mode
Current Consumption (mA)
MHz MEMS + MCU
MHz XTAL + MCU
Operating Mode 5.2 (7% lower) 5.6
Sleep Mode 2.1 (18% lower) 2.6
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µPower MEMS Clock Characteristics
SiT8021 - MHz clocks for MCU – Frequency range: 1 MHz to 26 MHz – Frequency stability: ± 50 ppm across -20⁰C to 70⁰C, VDD, aging – Core current = 60 uA – Standby current = 0.7 uA – Resume time = 2 ms – Footprint: 1.2mm2 (1508) – Drives multiple loads
SiT1552 – 32.768 kHz clocks for RTC/WD & BLE sleep clocks – Frequency range: 32.768 kHz – Frequency stability: ±5 ppm across temp, VDD, aging – Active current = 1 uA – Footprint: 1.2mm2 (1508)
– Drives multiple loads
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Smart Clocking Techniques
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Wearable Using µPower MEMS Clocks
• Eliminates 2x 32.768 kHz crystals & 4 load caps • Eliminates bulky MHz crystal & 2 load caps • Saves power and PCB real-estate
Synchronize GPIO with BLE radio state Optional
Low-Power
MCU
BLE SoC24 MHz
32 kHz 32 kHz
CLK
μPower
24 MHz
STB
VDD
GND
1.8V
μPower
32.768 kHz
CLKVDD
GND
1.8V
SPI/I2C
Antenna
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BLE: Power Saving Clocking Techniques
• Replace 32 kHz XTAL with µPower 5 ppm TCXO • Shut down on-chip XTAL oscillator
Shut-down RCX
• Shut-down 32 kHz RC (500 ppm) • Save CPU cycles used for calibration
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Dialog Semiconductor BLE: DA14580
MCU: Power Saving Clocking Techniques
• Replace with µpower MEMS oscillator
• Shut-down on-chip crystal oscillator
• MEMS reduces wake-up time
• Shut-down RC oscillator • Save CPU cycles used
for calibration
• Set RC oscillator to 2 MHz
• Disable calibration; save CPU cycles
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STMicroelectronics MCU: STM32L476G
Conclusion
µPower MEMS 32.768 kHz XO reduces BLE current drain by 30% – 32 kHZ TCXO optimizes BLE RX radio ON time
– Supports extended sleep time
µPower MHz MEMS XO reduce MCU power consumption by 17% – Radio synchronized turn-on/off achieves lower active and sleep current
Added benefits of using µPower MEMS oscillators – Accurate time-keeping
– Reduces BOM cost
– Saves PCB real-estate
– Achieves an environmentally resilient product
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Power Savings Demonstration: Dialog Semi BLE Platform
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Demonstration HW
BLE Master
32 kHz MEMS TCXO
DA14580 BLE Slave
HDR Current Monitor
TCXO adapter board
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Dialog Semi Pro kit
Demo Software: SmartSnippets-1
Sleep Time = 1.2s
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Demo Software: SmartSnippets-2
ON Time = 7.63 ms
Coulombs consumed
• SlaveSCA = 200 ppm • Coulombs consumed during ON time = 6.6 µC
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Demo Software: SmartSnippets-2
ON Time = 7 ms
Coulombs consumed
• SlaveSCA = 5 ppm • Coulombs consumed during ON time = 4.98 µC ; Power savings = 32%
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Appendix
White Paper Resources at: http://www.sitime.com/support/sitime-university
Extending Battery Life of Wireless Medical Devices
Abstract: To continuously monitor and upload vital data, wireless medical devices need long battery life and long-term connectivity to the cloud. This paper discusses the advantages of Bluetooth® low energy (BLE) and a new architecture using an ultra-small, high-accuracy sleep clock that wakes up only when vitals must be updated, thus allowing the medical device to stay in sleep mode longer to reduce power consumption and extend battery life. http://www.sitime.com/images/stories/applications/SiTime-Extending-Battery-Life-of-Wireless-Medical-Devices.pdf
MEMS Timekeeper Extends Standby Life of Mobile Devices Abstract: This paper discusses how to extend battery life with techniques such as shutting down functional blocks with the highest current drain and switching to suspend or sleep states, and use of unique power saving strategies – such as programmable output frequencies and programmable output drive swing levels –
that reduce power consumption of always ON clocks used in mobile devices. http://www.sitime.com/images/stories/applications/SiTime-MEMS-TimeKeeper-Extends-Life-in-Mobile-Devices-2013.pdf
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